Katie Holmes' favourite hair care product is "caviar shampoo" from Alterna Haircare, which she is the ambassador and co-founder of.

The 37-year-old actress - who was announced as the brand ambassador and co-owner of Alterna Haircare - has admitted the company's shampoo, which contains fish eggs extract, is her essential product because it "smells great" and leaves her hair feeling nourished and silky.

Speaking to Byrdie.co.uk about her go to product, the 'Dawson's Creek' star said: "The longer I've been in this business, the more I've been exposed to wonderful things. Alterna is very high on that list.

"I've been working with Alterna for a couple years now. I love the company; I love how it began with a hairstylist who's in this industry. Every product is made with wonderful ingredients - it's paraben free. Using products with good-for-you ingredients is like eating well. It's the same concept. Over time, it has this positive effect on you. I've seen it make a huge difference in my hair.

"I love all of these products, and I use all of them.

"With the Moisture Milk, you leave it in, and it gives you that shine without it ever feeling greasy.

"My absolute favourite, though, is the Caviar Shampoo. It smells great and makes me feel great."

"When I don't have much time I'll use the Alterna Caviar Anti-Aging Dry Shampoo and just throw it up - especially in the heat. It's really just about embracing all of these things that make your beauty routine easier, faster, and better."

And the Ohio-born star - who was married to Tom Cruise for six years before the couple divorced in 2012 and who have 10-year-old daughter Suri together - has revealed she doesn't follow fashion or beauty trends and feels she is "constantly" breaking style rules with her ensembles.

She explained: "I think I'm constantly breaking the rules. I don't really know what the season's trends are. So I'm just kind of winging it. I'm sure I break them all the time."